(1) Field of the Invention
The present invention relates generally to a system and method for measuring low amounts of water in natural gas, and in particular to the use of a single-mode laser, e.g., a quantum cascade laser, emitting light in the 5.01 to 5.03 micron wavelength range, and less preferably in the 5.07 to 5.09 micron wavelength range, capable of measuring water vapor at below 10 ppmv. While the system and method is distinguishable from prior art systems and methods in the capability to measure at these lower volumes, it will be understood that the system and method may also be used to measure water vapor at above 10 ppmv.
(2) Description of the Prior Art
Distribution of commercial natural gas (consisting primarily of methane, CH4), requires an extensive network of pipelines through which the gas flows under pressure. Depending on many factors, the gas stream can also contain varying levels of other components such as heavier hydrocarbons, carbon dioxide (CO2) and water (H2O), among others. Due to the physical size and extent of the distribution pipeline system, custody of the gas can change hands many times between the point of origin and the end customer (typically a municipal utility serving individual homes and businesses). Pipeline networks are owned and managed by dozens of companies in the U.S. (and a similar situation exists anywhere in the world where natural gas is produced and used), and extensive monitoring of the gas quality is performed at most custody transfer points (i.e. the point at which ownership of the pipeline section(s) changes).
In addition to measurements of basic gas quality, it is a requirement to also monitor for levels of certain gases for safety and pipeline integrity reasons. Each of these gases has certain tariff levels which cannot be exceeded. For example, levels of hydrogen cyanide (H2S) cannot exceed 4 parts per million by volume (ppmv) as it is a highly toxic gas (less toxic mercaptons are added to natural gas to create the “rotten egg” smell so that leaks inside a building can be detected by smell). Levels of H2O and CO2 are also controlled because these gases can corrode the interior of the high pressure pipelines either directly, or by forming acids which attack the interior pipeline walls. H2O levels, in particular, must be kept low (typically below 150 ppmv in mid latitude locations, and much lower in colder northern climates) to minimize corrosion potential. Therefore, all natural gas pipeline operators are required to perform continuous measurements of the water vapor content within their pipelines.
H2O is a difficult gas to measure within a natural gas mixture using conventional humidity or moisture monitoring techniques. Most of these sensors are in direct contact with the natural gas stream, and the small levels of contaminates in the natural gas eventually alter the characteristics of the sensor causing it to either fail, or drift unacceptably out of calibration. Therefore, maintenance costs are high and the sensors are unreliable.
In 1999 SpectraSensors, Inc. (San Dimas, Calif.) developed the first laser-based H2O sensors for natural gas which offered a reliable and accurate solution to the problem of H2O monitoring in natural gas for mid-latitude locations where H2O levels cannot exceed 150 ppmv, and lower detection limits are typically 5-10 ppmv. U.S. Pat. No. 6,657,198, issued Dec. 2, 2003, by the present inventor, describes and claims a system for detecting water vapor in natural gas that is comprised of a light source emitting light at a frequency substantially corresponding to an absorption line of water in the 1.877-1.901 .mu.m wavelength range, wherein said light source is positioned to absorb light through the natural gas; a detector configured to detect the intensity of light emitted from the light source; and electronics coupled to the detector for determining the level of water vapor in the natural gas.
U.S. Pat. No. 7,132,661, issued Nov. 7, 2006, also by the present inventor, describes and claims a system comprised of at least one chemical sensor to detect a level of water vapor in natural gas; at least one optical sensor to detect a level of water vapor in natural gas, the optical sensor comprising: a light source emitting light at substantially a single wavelength having a width sufficiently narrow to conduct single line spectroscopy and corresponding to a single absorption line at which water molecules absorb light at a substantially greater level than natural gas molecules; a detector configured to detect the intensity of light emitted from the light source; electronics coupled to the detector for determining the level of water vapor in the natural gas using single line harmonic spectroscopy; and a supply line delivering natural gas to the at least one chemical sensor and the at least one optical sensor for parallel measurements.
The '661 patent claims three specific wavelength regions where H2O could be monitored within a background of primarily CH4 (natural gas consists of 85-98% CH4). These wavelength regions (near 0.95 microns, 1.9 microns, and 2.7 microns) meet the need for mid-latitude tariff levels for H2O monitoring (i.e., below 150 ppmv). However, there are many locations in the U.S., Canada and other regions worldwide where it is necessary to monitor H2O at levels well below 10 ppmv.
Molecular absorption spectra consist of broad features called “bands.” Each band, in turn, is composed of up to thousands of individual closely-spaced “lines” that can only be observed with a high resolution spectrometer. Lines have no substructure, so if the resolution of the light source is sufficient to observe a single line without distortion, it is possible to separate an H2O line from a CH4 line in a gas mixture which contains both gases. The trick then is to locate an H2O line that is separated sufficiently from the neighboring CH4 lines so that it can be isolated and monitored.
Molecular spectroscopy involves the detection of specific molecules using their unique absorption spectrum, and related studies of the physics behind such light absorption. This technique has been utilized for over 100 years to monitor gases, and “resolution” is a key concept to understand when comparing various approaches to molecular spectroscopy, and in particular why lasers offer many advantages over more conventional techniques. In all molecular spectroscopy methods, a light source is directed through the sample gas of interest to impinge on a detector. As the light passes through the gas sample, certain wavelengths may be absorbed while others pass through, with each gas producing a unique pattern (spectrum) when the detector signal is analyzed as a function of the wavelength of the light source.
In order to “split” the wavelength of a conventional light source (e.g. the sun, a hot filament, etc.) into components it is necessary to use a dispersive element such as a grating. This is analogous to how a prism separates sunlight into visible colors, but instead of a few colors that the human eye can easily see in the prism rainbow, a grating may split the light into hundreds or even thousands of closely-separated wavelengths. The more wavelengths that can be distinguished, the higher the “resolution” of the system, and gratings can produce this wavelength separation throughout the electromagnetic spectrum (visible, infrared, etc.). In this case the light source itself produces a wide, continuous range of wavelengths, and the dispersive element separates that light into many equally-spaced wavelengths. Similar to the teeth on a comb, the higher the resolution the more teeth on the comb, and the more closely they are spaced. The total width of the comb represents the total wavelength region covered by the light source. A low resolution spectrometer (an instrument that measures a molecular absorption spectrum) would have only a few big teeth on the comb, while a high resolution spectrometer would have many closely spaced teeth on the comb. The higher the resolution of the spectrometer, the greater the ability to separate the absorption spectrum into its fine details. Conventional spectrometers have a limit to how many teeth can be created on the comb analogy, but lasers do not.
When the laser was invented in the 1960s, it changed molecular spectroscopy in a major way because it was finally possible to record an absorption spectrum with “infinite” resolution. The spectral purity of the laser output (i.e., one and only one very precise wavelength) eliminates the need for a grating or dispersive element, and there are many laser sources which are continuously tunable over certain wavelength intervals. This means that the single, pure wavelength that the laser outputs can be smoothly tuned over a small wavelength interval. Such lasers are called continuous wave (CW), tunable, single-mode lasers.
While the systems and methods described in the foregoing patents are useful in measuring water levels at higher and mid-levels, there is a continuing need for a simple and effective system and method that would be useful in the measurement of water levels at below 10 ppmv.